Siloxane polymerization in wallboard

ABSTRACT

Polymerization of siloxane is improved using a gypsum-based slurry that includes stucco, Class C fly ash, magnesium oxide and an emulsion of siloxane and water. This slurry is used in a method of making water-resistant gypsum articles that includes making an emulsion of siloxane and water, then combining the slurry with a dry mixture of stucco, magnesium oxide and Class C fly ash. The slurry is then shaped as desired and the stucco is allowed to set and the siloxane polymerizes. The resulting product is useful for making a water-resistant gypsum panel having a core that includes interwoven matrices of calcium sulfate dihydrate crystals and a silicone resin, where the interwoven matrices have dispersed throughout them a catalyst comprising magnesium oxide and components from a Class C fly ash.

BACKGROUND OF THE INVENTION

This invention relates to a method for making water resistant gypsumproducts that include siloxane. More specifically, the present inventionrelates to the addition of a novel catalyst for curing of the siloxanein a gypsum product.

Gypsum-based building products are commonly used in construction.Wallboard made of gypsum is fire retardant and can be used in theconstruction of walls of almost any shape. It is used primarily as aninterior wall and ceiling product. Gypsum has sound-deadeningproperties. It is relatively easily patched or replaced if it becomesdamaged. There are a variety of decorative finishes that can be appliedto the wallboard, including paint and wallpaper. Even with all of theseadvantages, it is still a relatively inexpensive building material.

Gypsum is also known as calcium sulfate dihydrate, terra alba orlandplaster. Plaster of Paris is also known as calcined gypsum, stucco,calcium sulfate semihydrate, calcium sulfate half-hydrate or calciumsulfate hemihydrate. Synthetic gypsum, which is a byproduct of flue gasdesulfurization processes from power plants, may also be used. When itis mined, raw gypsum is generally found in the dihydrate form. In thisform, there are approximately two water molecules of water associatedwith each molecule of calcium sulfate. In order to produce thehemihydrate form, the gypsum can be calcined to drive off some of thewater of hydration by the following equation:CaSO₄.2H₂O→CaSO₄.1/2H₂O+3/2H₂O

A number of useful gypsum products can be made by mixing the stucco withwater and permitting it to set by allowing the calcium sulfatehemihydrate to react with water to convert the hemihydrate into a matrixof interlocking calcium sulfate dihydrate crystals. As the matrix forms,the product slurry becomes firm and holds a desired shape. Excess watermust then be removed from the product by drying.

In the absence of additives to prevent it, set gypsum absorbs up to 50%of its weight when immersed in water. Boards or panels that absorb waterswell, become deformed and lose strength. This property is undesirablein products that are likely to be exposed to water. In areas such asbathrooms or kitchens, high temperature and humidity are common, andwalls are likely to be splashed. In such areas, it is preferable to usea gypsum board that exhibits water resistancy, thus maintaining strengthand dimensional stability.

Many attempts have been made to improve the water resistance of gypsumproducts. Various hydrocarbons, including wax, resins and asphalt havebeen added to the slurry in order to impart water resistance to the setproduct. The use of siloxanes, which form silicone resins in gypsumproducts, to impart water resistance is well known.

Although the use of siloxanes in gypsum slurries is a useful means ofimparting water resistance to the finished product, there are drawbacksassociated with it. When added to a gypsum slurry to form siliconeresins in situ, siloxane can be slow to cure. The siloxane forms areactive silanol intermediate to yield polymethylsilicic acid, whichcross links to form the silicone resin. The reaction proceeds slowly,often continuing after the gypsum is set and requiring one to two weeksto fully develop water-resistance. Wallboard made using this method mustbe stored for a time sufficient for the water-resistance to developbefore the board can be shipped. In some cases, the siloxane may notcure within a reasonable time or it may not cure fully. In such cases,the water resistance does not develop in the gypsum board to asatisfactory level. Additionally, failure to cure fully leads to using alarger dose of the siloxane, increasing the cost of the raw materials.

Catalysts, such as alkaline earth oxides and hydroxides, are known toaccelerate the curing reaction of siloxane in a stucco slurry. Thesecatalysts are relatively water soluble and elevate the pH of the slurry.High pH can interfere with the rehydration of the stucco, and cannegatively react with some preferred wallboard additives. Thus, whilethe siloxane polymerization is promoted, other considerations make theuse of these catalysts undesirable.

Magnesium oxide (“MgO”) is known to catalyze siloxane reactions, butwhere the catalyst reactivity is high enough to fully cure the siloxane,undesirable cracking results. Light-burned MgO has the activity neededto cure siloxane quickly, but the activity leads to unwanted sidereactions. These side reactions generate hydrogen, which cause expansionof the product and cracking of set gypsum. Hard-burned or dead-burnedMgO has lower reactivity, but results in a less water-resistant product.Thus, when MgO is used alone, it is very difficult to balance catalystactivity with the desired extent of siloxane polymerization.

There are also certain stucco sources for which it is very difficult todrive the polymerization of siloxane. Gypsum is a complex mixture ofcalcium sulfate in various forms, salts and a variety of aluminates,silicates and aluminosilicates. Apparently some gypsum sources includeone or more components that suppress the formation of the siliconeresin. When used with these stuccos, known catalysts fall short of thedesired level of water-resistance of less than 5% water absorbance.

Thus there is a need in the art for a catalyst and a method of producingwater-resistant gypsum articles with improved water-resistance atreasonable cost. The catalyst should be relatively inexpensive, havinggood activity for siloxane polymerization with a minimum of unwantedside reactions. There should be little interference between the catalystand other common gypsum additives.

SUMMARY OF THE INVENTION

These and other needs are met or exceeded by the present invention whichaccelerates the polymerization of siloxane and in some cases reduces theamount of siloxane needed to meet the specifications of ASTM 1398.

More specifically, polymerization of siloxane is improved using a slurrythat includes stucco, Class C fly ash, magnesium oxide and an emulsionof siloxane and water. This slurry is used in a method of makingwater-resistant gypsum articles that includes making an emulsion ofsiloxane and water, then combining the slurry with a dry mixture ofstucco, magnesium oxide and Class C fly ash. The slurry is then shapedas desired and the stucco is allowed to set and the siloxanepolymerizes.

The resulting product is useful for making a water-resistant gypsumpanel having a core that includes interwoven matrices of calcium sulfatedihydrate crystals and a silicone resin, where the interwoven matriceshave dispersed throughout them a catalyst comprising magnesium oxide andcomponents from a Class C fly ash.

The mixture of magnesium oxide and Class C fly ash catalyzes thepolymerization of siloxane to accelerates development ofwater-resistance in product made from the slurry. Water-resistantproducts such as wallboard need not be stored for lengthy periods oftime awaiting completion of the polymerization reactions of thesiloxane.

Use of this catalyst also increases the extent of the reaction, leadingto improved water-resistance. Water absorption of less than 5% by weightwas attainable using the fly ash and magnesia combination, where it hadnot been achieved with either catalyst alone. Thus, in addition tocausing the polymerization reaction to accelerate, this catalyst alsoallows the siloxane to polymerize more completely allowing the amount ofsiloxane to be reduced in some cases. Since the siloxane is one of themore expensive wallboard additives, reduction in the dosage leads to asavings in the cost of the raw materials.

Another advantage of the present invention is the dimensional stabilityof the product. Some compounds used to catalyze this reaction result insignificant expansion as the product dries. As the board interiorexpands, it causes cracking in the exterior board surface, damaging it.Use of fly ash and magnesium oxide results in very little expansion andvery little cracking in the finished product.

This combined fly ash and magnesia catalyst also allows for satisfactorypolymerization using a wider range of magnesium oxide grades. While theprior art discloses only that dead-burned magnesia is suitable to act asa catalyst for siloxane polymerization, when combined with fly ash, evenhard-burned or light-burned magnesium oxide may be used. This featureallows manufacturers of gypsum products additional freedom in selectionsources of magnesium oxide to be used in the slurry.

DETAILED DESCRIPTION OF THE INVENTION

The present invention broadly contemplates improving the waterresistance of gypsum based articles by adding a polymerizable siloxaneto the slurry used to make the gypsum based articles. Preferably, thesiloxane is added in the form of an emulsion. The slurry is then shapedand dried under conditions which promote the polymerization of thesiloxane to form a highly cross-linked silicone resin. A catalyst whichpromotes the polymerization of the siloxane to form a highlycross-linked silicone resin is added to the gypsum slurry.

Preferably, the siloxane is generally a fluid linear hydrogen-modifiedsiloxane, but can also be a cyclic hydrogen-modified siloxane. Suchsiloxanes are capable of forming highly cross-linked silicone resins.Such fluids are well known to those of ordinary skill in the art and arecommercially available and are described in the patent literature.Typically, the linear hydrogen modified siloxanes useful in the practiceof the present invention comprise those having a repeating unit of thegeneral formula:

wherein R represents a saturated or unsaturated mono-valent hydrocarbonradical. In the preferred embodiments, R represents an alkyl group andmost preferably R is a methyl group. During polymerization, the terminalgroups are removed by condensation and siloxane groups are linkedtogether to form the silicone resin. Cross-linking of the chains alsooccurs. The resulting silicone resin imparts water resistance to thegypsum matrix as it forms.

The gypsum-based water resistant articles of the present invention arepreferably made with a solventless methyl hydrogen siloxane fluid soldunder the name SILRES BS 94 by Wacker-Chemie GmbH (Munich, Germany) asthe siloxane. The manufacturer indicates this product is a siloxanefluid containing no water or solvents. It is contemplated that about 0.3to 1.0% of the BS 94 siloxane may be used, based on the weight of thedry ingredients. It is preferred to use from about 0.4 to about 0.8% ofthe siloxane based on the dry stucco weight.

After the slurry is formed, the siloxane is formed into an emulsion or astable suspension with water. A number of siloxane emulsions arecontemplated for use in this slurry. Emulsions of siloxane in water arealso available for purchase, but they may include emulsifying agentsthat tend to modify properties of the gypsum articles, such as the paperbond in wallboard products. Emulsions or stable suspensions preparedwithout the use of emulsifiers are therefore preferred. Preferably, asuspension is formed in situ by mixing the siloxane fluid with water. Itis essential that the siloxane suspension be stable until it reaches thepin mixer and that it remain well dispersed under the conditions of theslurry. The siloxane suspension or emulsion must remain well dispersedin the presence of the optional additives, such as set accelerators,that are present in the slurry. The siloxane suspension or emulsion mustalso remain stable through the steps in which the gypsum based articlesare formed as well. Preferably, the suspension remains stable for morethan 40 minutes. More preferably, it remains stable for at least onehour. In the discussion and claims that follow, the term “emulsion” isintended to include true emulsions and suspensions that are stable atleast until the stucco is 50% set.

In a preferred embodiment, at least a portion of the gauging water iscontinuously fed to the high shear mixer. Siloxane fluid is metered intothe high shear mixer with the water to form the emulsion in 1-2 seconds.The proportion of water to siloxane is not critical and a mixture of 25parts water to one part siloxane is known to be useful. This emulsion isstable for several minutes without the addition of an emulsifier, longenough to mix the slurry, form the article and allow it to start to set.In the alternative, use of a portion of the gauging water to form theemulsion is also contemplated. A slip stream of the gauging water iscombined with the siloxane in the high shear mixer. The siloxaneemulsion is then preferably added to the gauging water before the slurryis formed to provide sufficient time for the siloxane emulsion tothoroughly mix with water used to form the slurry and be uniformlydispersed throughout the resulting articles.

While not wishing to be bound by theory, it is believed that waterresistance develops when the siloxane cures within the formed wallboard.The polymerization reaction proceeds slowly on its own, requiring thatthe wallboard be stored for a time sufficient to developwater-resistance prior to shipping. Catalysts are known to acceleratethe polymerization reaction, reducing or eliminating the time needed tostore wallboard product as the water-resistance develops. Use ofdead-burned magnesium oxide for siloxane polymerization is described inco-pending U.S. Ser. No. 10/917,177, entitled “Method of MakingWater-Resistant Gypsum-Based Article”, herein incorporated by reference.Dead-burned magnesium oxide is water-insoluble and interacts less withother components of the slurry. It accelerates curing of the siloxaneand, in some cases, causes the siloxane to cure more completely. It iscommercially available with a consistent composition. A particularlypreferred source of dead-burned magnesium oxide is BAYMAG 96. It has aBET surface area of at least 0.3 m²/g. The loss on ignition is less than0.1% by weight. The magnesium oxide is preferably used in amounts ofabout 0.1 to about 0.5% based on the dry stucco weight.

There are at least three grades of magnesium oxide on the market,depending on the calcination temperature. “Dead-burned” magnesium oxideis calcined between 1500° C. and 2000° C., eliminating most, if not all,of the reactivity. MagChem P98-PV (Martin Marietta Magnesia Specialties,Bethesda, Md.) is an example of a “dead burned” magnesium oxide. BayMag96 (Baymag, Inc. of Calgary, Alberta, Canada) and MagChem 10 (MartinMarietta Magnesia Specialties, Bethesda, Md.) are examples of“hard-burned” magnesia. “Hard-burned” magnesium oxide is calcined attemperatures from 1000° C. to about 1500° C. It has a narrow range ofreactivity, a high density, and is normally used in application whereslow degradation or chemical reactivity is required, such as in animalfeed and fertilizer. The third grade is “light-burn” or “caustic”magnesia, produced by calcining at temperatures of about 700° C. toabout 1000° C. This type of magnesia is used in a wide range ofapplications, including plastics, rubber, paper and pulp processing,steel boiler additives, adhesives and acid neutralization. Examples oflight burned magnesia include BayMag 30, BayMag 40, and BayMag 30 (−325Mesh) (BayMag, Inc. of Calgary, Alberta, Canada).

It has been discovered that preferred catalysts are made of a mixture ofmagnesium oxide and Class C fly ash. When combined in this manner, anyof the grades of magnesium oxide are useful. However, dead-burned andhard-burned magnesium oxides are preferred due to reduced reactivity.The relatively high reactivity of magnesium oxides, can lead to crackingreactions which can produce hydrogen. As the hydrogen is generated, theproduct expands, causing cracks where the stucco has set. Expansion alsocauses breakdown of molds into which the stucco is poured, resulting inloss of detail and deformation of the product in one or more dimensions.Preferably, BayMag 96, MagChem P98-PV and MagChem 10 are the preferredsources of magnesium oxide. Preferably, the magnesium oxide and fly ashare added to the stucco prior to their addition to the gauging water.Dry components such as these are often added to the stucco as it movesalong a conveyer to the mixer.

A preferred fly ash is a Class C fly ash. Class C hydraulic fly ash, orits equivalent, is the most preferred fly ash component. A typicalcomposition of a Class C fly ash is shown in Table 1. High lime contentfly ash, greater than 20% lime by weight, which is obtained from theprocessing of certain coals. ASTM designation C-618, herein incorporatedby reference, describes the characteristics of Class C fly ash. Apreferred Class C fly ash is supplied by Bayou Ash Inc., Big Cajun, II,LA. Preferably, fly ash is used in amounts of about 0.1% to about 5%based on the dry stucco weight. More preferably, the fly ash is used inamounts of about 0.2% to 1.5% based on the dry stucco weight. TABLE ITypical Type C Fly Ash Composition Composition Amount, wt % SiO₂ 25-59Al₂O₃ 14-22 Fe₂O₃  5-13 CaO  8-32 MgO  3.2-12.5 K₂O 0.3-1.6 Na₂O 0.8-6.0SO₃ 0.4-5.0 TiO₂ <1.0 Loss On Ignition 0.1-2.3

Catalysis of the siloxane results in faster and more completepolymerization and cross-linking of siloxane to form the silicone resin.Hydration of the stucco forms an interlocking matrix of calcium sulfatedihydrate crystals. While the gypsum matrix is forming, the siloxanemolecules are also forming a silicone resin matrix. Since these areformed simultaneously, at least in part, the two matrices becomeintertwined in each other. Excess water and additives to the slurry,including the fly ash, magnesium oxide and additives described below,which were dispersed throughout the slurry, become dispersed throughoutthe matrices in the interstitial spaces.

When used to make gypsum board, a number of additives are useful toimprove the properties of the finished article. Traditional amounts ofadditives are used. Except as noted, there are no known interactions ofthe catalyst or polysiloxane that interferes with the additives. Amountsof several additives are reported as “lbs/MSF,” which stands for poundsof additive per one thousand square feet of board.

Some embodiments of the invention employ a foaming agent to yield voidsin the set gypsum-containing product to provide lighter weight. In theseembodiments, any of the conventional foaming agents known to be usefulin preparing foamed set gypsum products can be employed. Many suchfoaming agents are well known and readily available commercially, e.g.the HYONIC line of soaps from GEO Specialty Chemicals, Ambler, Pa. Foamsand a preferred method for preparing foamed gypsum products aredisclosed in U.S. Pat. No. 5,683,635, herein incorporated by reference.

Dispersants are used to improve the flowability of the slurry and reducethe amount of water used to make the slurry. Any known dispersant isuseful, including polycarboxylates, sulfonated melamines or naphthalenesulfonate. Naphthalene sulfonate is the most preferred dispersant, andis used in amounts of about 0 lb/MSF to 18 lb/MSF, preferably from about4 lb/MSF to about 12 lb/MSF. A preferred naphthalene sulfonatedispersant is DAXAD Dispersant (Dow Chemical, Midland, Mich.)

Water is added to the slurry in any amount that makes a flowable slurry.The amount of water to be used varies greatly according to theapplication with which it is being used, the exact dispersant beingused, the properties of the stucco and the additives being used. Thewater to stucco ratio (“WSR”) for wallboard is preferably about 0.2 toabout 1.2 based on the dry weight of the stucco. Commonly, a WSR ofabout 0.4 to about 0.9 is preferred. Water used to make the slurryshould be as pure as practical for best control of the properties ofboth the slurry and the set plaster. Salts and organic compounds arewell known to modify the set time of the slurry, varying widely fromaccelerators to set inhibitors. Some impurities lead to irregularitiesin the structure as the interlocking matrix of dihydrate crystals forms,reducing the strength of the set product. Product strength andconsistency is thus enhanced by the use of water that is ascontaminant-free as practical.

The stucco, also known as calcium sulfate hemihydrate or calcinedgypsum, is present in amounts of at least 50% of the dry materials.Preferably, the amount of stucco is at least 80%. In many wallboardformulations, the dry component material is more than 90% or even 95%calcium sulfate hemihydrate. The method of calcination is not important,and either alpha or beta-calcined stucco is suitable. Use of calciumsulfate anhydrite is also contemplated, although it is preferably usedin small amounts of less than 20%.

A trimetaphosphate compound is added to the gypsum slurry in someembodiments to enhance the strength of the product and to improve sagresistance of the set gypsum. Preferably the concentration of thetrimetaphosphate compound is from about 0.07% to about 2.0% based on theweight of the calcined gypsum. Gypsum compositions includingtrimetaphosphate compounds are disclosed in U.S. Pat. Nos. 6,342,284 and6,632,550, both herein incorporated by reference. Exemplarytrimetaphosphate salts include sodium, potassium or lithium salts oftrimetaphosphate, such as those available from Astaris, LLC., St. Louis,Mo. Care must be exercised when using trimetaphosphate with lime orother modifiers that raise the pH of the slurry. Above a pH of about9.5, the trimetaphosphate looses its ability to strengthen the productand the slurry becomes severely retardive.

Other additives are also added to the slurry as are typical for theparticular application to which the gypsum slurry will be put. Setretarders (up to about 2 lb./MSF (9.8 g/m2)) or dry accelerators (up toabout 35 lb./MSF (170 g/m2)) are added to modify the rate at which thehydration reactions take place. “CSA” is a set accelerator comprising95% calcium sulfate dihydrate co-ground with 5% sugar and heated to 250°F. (121° C.) to caramelize the sugar. CSA is available from USGCorporation, Southard, Okla. plant, and is made according to U.S. Pat.No. 3,573,947, herein incorporated by reference. Potassium sulfate isanother preferred accelerator. HRA is calcium sulfate dihydrate freshlyground with sugar at a ratio of about 5 to 25 pounds of sugar per 100pounds of calcium sulfate dihydrate. It is further described in U.S.Pat. No. 2,078,199, herein incorporated by reference. Both of these arepreferred accelerators.

Another accelerator, known as wet gypsum accelerator or WGA, is also apreferred accelerator. A description of the use of and a method formaking wet gypsum accelerator are disclosed in U.S. Pat. No. 6,409,825,herein incorporated by reference. This accelerator includes at least oneadditive selected from the group consisting of an organic phosphoniccompound, a phosphate-containing compound or mixtures thereof. Thisparticular accelerator exhibits substantial longevity and maintains itseffectiveness over time such that the wet gypsum accelerator can bemade, stored, and even transported over long distances prior to use. Thewet gypsum accelerator is used in amounts ranging from about 5 to about80 pounds per thousand square feet (24.3 to 390 g/m²) of board product.

Other potential additives to the wallboard are biocides to reduce growthof mold, mildew or fungi. Depending on the biocide selected and theintended use for the wallboard, the biocide can be added to thecovering, the gypsum core or both. Examples of biocides include boricacid, pyrithione salts and copper salts. Biocides can be added to eitherthe covering or the gypsum core. When used, biocides are used in thecoverings in amounts of less than 500 ppm. Pyrithione is known byseveral names, including 2-mercaptopyridine-N-oxide;2-pyridinethiol-1-oxide (CAS Registry No. 1121-31-9);1-hydroxypyridine-2-thione and 1 hydroxy-2(1H)-pyridinethione (CASRegistry No. 1121-30-8). The sodium derivative (C₅H₄NOSNa), known assodium pyrithione (CAS Registry No. 3811-73-2), is one embodiment ofthis salt that is particularly useful. Pyrithione salts are commerciallyavailable from Arch Chemicals, Inc. of Norwalk, Conn., such as SodiumOMADINE or Zinc OMADINE.

In addition, the gypsum composition optionally can include a starch,such as a pregelatinized starch or an acid-modified starch. Starches areused in amounts of from about 3 to about 20 lbs/MSF (14.6 to 97.6 g/m²)to increase paper bond and strengthen product. The inclusion of thepregelatinized starch increases the strength of the set and dried gypsumcast and minimizes or avoids the risk of paper delamination underconditions of increased moisture (e.g., with regard to elevated ratiosof water to calcined gypsum). One of ordinary skill in the art willappreciate methods of pregelatinizing raw starch, such as, for example,cooking raw starch in water at temperatures of at least about 185° F.(85° C.) or other methods. Suitable examples of pregelatinized starchinclude, but are not limited to, PCF 1000 Starch, commercially availablefrom Lauhoff Grain Company and AMERIKOR 818 and HQM PREGEL starches,both commercially available from Archer Daniels Midland Company. Ifincluded, the pregelatinized starch is present in any suitable amount.For example, if included, the pregelatinized starch can be added to themixture used to form the set gypsum composition such that it is presentin an amount of from about 0.5% to about 10% percent by weight of theset gypsum composition. Starches such as USG95 (United States GypsumCompany, Chicago, Ill.) are also optionally added for core strength.

Other known additives may be used as needed to modify specificproperties of the product. Sugars, such as dextrose, are used to improvethe paper bond at the ends of the boards. Wax emulsions or siloxanes areused for water resistance. If stiffness is needed, boric acid iscommonly added. Fire retardancy can be improved by the addition ofvermiculite. These and other known additives are useful in the presentslurry and wallboard formulations. Glass fibers are optionally added tothe slurry in amounts of up to 11 lb./MSF (54 g/m²). Up to 15 lb./MSF(73.2 g/m²) of paper fibers are also added to the slurry. Wax emulsionsare added to the gypsum slurry in amounts up to 90 lb./MSF (0.439 kg/m²)to improve the water-resistency of the finished gypsum board panel.

In operation, a slip stream is taken from the gauging water and combinedwith siloxane and water in a high shear mixer to form the siloxaneemulsion. The two components are mixed for several minutes until astabile emulsion is formed. From the high shear mixer, the emulsion goesdirectly to the slurry mixer where it is combined with the remainder ofthe gauging water.

Meanwhile, the stucco is moved toward a slurry mixer. Prior to entryinto the mixer, dry additives, such as starches, or set accelerators,are added to the powdered stucco. Some additives are added directly tothe mixer via a separate line. For most additives, there is nocriticality regarding placing the additives in the slurry, and they maybe added using whatever equipment or method is convenient.

After mixing, wallboard optionally has foam added to decrease theproduct density. Foam is generated by combining soap and water. The foamis then injected into the moving gypsum slurry after it exits from themixer through a hose or chute. The foam ring is an apparatus havingmultiple ports that are arranged in a ring perpendicular to the axis ofthe hose so that foam is forced under pressure into the gypsum slurry asit passes by the foam ring.

When the foam and the slurry have been brought together, the resultingslurry moves toward and is poured onto a conveyor lined with one pieceof facing material. Another piece of facing material is placed on top ofthe slurry, forming a sandwich with the slurry between the two facingmaterials. The sandwich is fed to a forming plate, the height of whichdetermines the thickness of the board. Next the continuous sandwich iscut into appropriate lengths at the cutting knife, usually eight feet totwelve feet.

The boards are then moved to a kiln for drying. Temperatures in the kilntypically range to 450° F. to 500° F. maximum. Preferably there arethree or more temperature zones in the kiln. In the first zone contactedby the wet board, the temperature increases to the maximum temperature,while the temperature slowly decreases in the last two zones. The blowerfor the first zone is positioned at the exit of the zone, blowing theair countercurrent to the direction of board travel. In the second andthird zones, the blowers are located at the entrance to the zone,directing the hot air co-current with board travel. Heating that is lesssevere in the last zone prevents calcination of dry areas of the board,causing poor paper bond. A typical residence time in the kiln is aboutforty minutes, but the time will vary depending on the line capacity,the wetness of the board and other factors.

EXAMPLE 1

Two grams of BAYMAG 30 dead-burned magnesium oxide and 4 grams of ClassC fly ash were added to 500 grams of stucco from each of several sourcesspecified in Table II. These dry components were placed in a plastic bagand shaken to mix them. An emulsion was prepared by mixing 3.2 grams ofBS 94 siloxane and 550 grams of water for 2.5 minutes in a high shearSilverson Mixer (Silverson Machines, East Longmeadow, Mass.). Theemulsion was transferred to a 1 liter Waring blender at 7500 rpm (WaringProducts, Inc., Torrington, Conn.) for 10 seconds. The slurry was thencast into a 2″×2″×2″ cube mold. After set, the cubes were unmolded andplaced in an oven for drying to a constant weight at 110° F. Dried cubeswere soaked in water for two hours for the water absorption test asspecified in ASTM C1396, herein incorporated by reference. The weightgain during the soaking was used to calculate the water absorption.TABLE II Stucco Dry Dry Wet Water % Source Density Weight Weight PickupAbsorption Empire 43.0 90.27 93.44 3.17 3.5 ″ 42.6 89.5 92.8 3.31 3.7 ″42.8 89.8 93.1 3.29 3.7 Montreal 43.4 91.26 93.71 2.45 2.7 ″ 43.5 91.3893.97 2.59 2.8 ″ 43.3 91.1 93.5 2.46 2.7 Sperry 42.8 89.89 93.89 4.0 4.4″ 42.5 89.4 92.9 3.55 4.0 ″ 42.6 89.6 93.3 3.74 4.2 Sweet Water 43.892.09 96.51 4.42 4.8 ″ 43.5 91.4 95.6 4.12 4.5 ″ 43.7 91.8 96.1 4.31 4.7

This example demonstrates the ability of this catalyst combination toreduce water absorption to less than 5% in a variety of stuccos.

EXAMPLE 2

Commercial scale trials where held to test the behavior of a slurryincluding b 12 lb/MSF siloxane, and having magnesium oxide and fly ashas indicated in Table III. The fly ash and magnesium oxide were added tothe stucco prior to entering the mixer. The brand and type of calcinedmagnesium oxide is also shown in Table III.

Siloxane was added to the gauging water and mixed in a high shear mixerto form an emulsion. This emulsion and the dry components were combinedin the stucco mixer until a homogeneous slurry was formed, and theslurry was deposited on a face paper on a conveyor. Backing paper wasplaced atop the slurry and the sandwich was fed to a forming roller thatflattened the sandwich to a uniform ½inch (1.2 cm) thickness. When theslurry was sufficiently set to hold its shape, the continuous board wascut into 8-foot lengths. TABLE III Trial Condition Trial Board MgO MgOFlyash 2 Hr. Source Amount Amount Absorption Baymag 96 4 0 5.3% ″ 4 05.9% ″ 4 0 4.4% ″ 4 0 5.4% ″ 4 0 5.8% MagChem 10 4 0 5.2% ″ 4 0 5.2% ″ 40 5.5% ″ 4 0 5.7% ″ 4 0 5.7% ″ 4 10 4.3% ″ 4 10 4.6% ″ 4 10 4.3% ″ 4 103.9% ″ 4 10 4.5% Baymag 30 3 10 4.4% ″ 3 10 4.4% ″ 3 10 4.6% ″ 3 10 4.6%″ 3 10 4.4% ″ 3 0 5.7% ″ 3 0 5.8% ″ 4 0 5.6% ″ 4 0 6.2% ″ 4 0 5.8% ″ 4 07.1%

When fly ash was added to MagChem 10 dead-burned magnesium oxide, thewater resistance improved more than 25%. The same comparison holds whenthe same amount of fly ash was added to 3 lb/MSF of BayMag 30. Thecombination of fly ash with 3 lb/MSF BayMag 30 also performs better than4 lb/MSF of BayMag 30

EXAMPLE 3

Cubes were made according to Example 1 using Shoals stucco and 0.6 wt %BS 94 siloxane. Either Baymag 30 magnesium oxide, fly ash or both wasadded to the slurry as indicated in Table IV. The target wateradsorption was 6%. TABLE IV Baymag 30 Fly Ash Water Adsorption 0.4% 021.5% 0 0.8%   24% 0.4% 0.8%  2.0%

When magnesium oxide and fly ash are used together, water reductionimproved by an order of magnitude in the above example.

EXAMPLE 4

Gypsum cubes were made according to the method of Example 1 using thesiloxane dosage and catalyst composition shown in Table 4. Results ofwater absorption tests are also shown in Table V. TABLE V StuccoSiloxane Fly Water Source Dosage MgO Ash Absorption Empire 4.2 g 1.2 g 06.1% ″ 4.2 g 0 6.0 g  32% ″ 3.1 g 1.2 g 6.0 g 3.7% Montreal 4.2 g 1.2 g0   7% ″ 4.2 g 0 6.0 g  40% ″ 3.1 g 1.2 g 6.0 g 2.9%

These tests were preformed on stuccos with which it is particularlydifficult to obtain satisfactory water resistance. Neither fly ash norMgO alone were able to produce the desired standard of less than 5%water absorption. However, when both catalysts were used together,absorption well below the standard was achieved, even with a lowerdosage of siloxane.

EXAMPLE 5

A plant trial was held testing this catalyst in wallboard on acommercial scale. The composition of the wallboard is shown in Table VI.TABLE VI Component Amount, lbs/MSF Stucco 1324 Gauging Water 546Siloxane Water 119 Foam Water 75 Siloxane 10.5 MgO 4 Fly Ash 10.85 Soap0.4 HRA Set Accelerator 16.6 Trimetaphosphate 0.8 LC-211 3.0 USG 95Starch 3.5 Thickener 0.49 Daxad Dispersant 5.5 Foam Air 17 ft³/MSF

HRA, Trimetaphosphate, USG95, thickener, Daxad, LC-211, Fly ash and MgOwere added to the dry stucco. The siloxane water and siloxane were mixedin a high speed mixer at high speed for less than 1 minute to make astable suspension of siloxane in water. The suspension was then pumpedto the slurry mixer and combined with the gauging water, thecatalyst/stucco blend. Residence in the mixer was less than 15 seconds.As the slurry was discharged from the mixer, foam, made of the soap,foam air and foam water was inserted into the slurry to reduce theproduct density. TABLE VI Sample Water Absorption 1A 4.11% 1B 4.36% 2A4.34% 2B 4.37% 3A 4.19% 4B 4.06%

Cubes were made from a slurry sample according to ASTM C1396. Results ofthe soak tests are shown in Table VI. These tests confirm that wallboardhaving less than 5% water absorption are producable in a commercialsetting using the catalyst, slurry and method of this invention.

While a particular embodiment of the fly ash and magnesium oxidecatalyst for siloxane polymerization has been shown and described, itwill be appreciated by those skilled in the art that changes andmodifications may be made thereto without departing from the inventionin its broader aspects and as set forth in the following claims.

1. A slurry comprising: stucco; Class C fly ash; magnesium oxide; and anemulsion of siloxane and water.
 2. The slurry of claim 1 wherein theratio of said fly ash to said magnesium oxide is from about 2:1 to about3:1.
 3. The slurry of claim 2 wherein said fly ash is present in theslurry in amounts of about 0.1% to about 5% based on the dry stuccoweight.
 4. The slurry of claim 1 wherein said fly ash is present inamounts ranging from about 0.1% to about 5% based on the dry stuccoweight.
 5. The slurry of claim 1 wherein said magnesium oxide is presentin amounts of about 0.1 to about 0.5% based on the dry stucco weight. 6.The slurry of claim 1 wherein said magnesium oxide is a hard-burned or adead-burned magnesium oxide.
 7. The slurry of claim 1 wherein saidemulsion comprises a polysiloxane fluid and water.
 8. The slurry ofclaim 1 wherein said siloxane is a fluid, linear, hydrogen-modifiedsiloxane.
 9. The slurry of claim 1 further comprising at least one ofthe group consisting of starches, foaming agents, set accelerators, setretarders, biocides, dispersants, fibers or strength enhancers.
 10. Theslurry of claim 1 wherein said emulsion comprises a stable suspension.11. The slurry of claim 1 wherein said suspension is emulsifier-free.12. A method of making a water-resistant gypsum article comprising:making a siloxane emulsion with siloxane and water; mixing magnesiumoxide and Class C fly ash with stucco; combining the siloxane emulsionwith the stucco/catalyst mixture; shaping the slurry; allowing thegypsum slurry to set, forming the wallboard core; and polymerizing thesiloxane.
 13. The method of claim 12 wherein said making step comprisesmixing the siloxane and water in a high shear mixer.
 14. The method ofclaim 12 wherein said shaping step comprises sandwiching the slurrybetween two pieces of facing material to form a wallboard panel.
 15. Themethod of claim 12 therein said mixing step takes place before saidcombining step.
 16. The method of claim 11 further comprising taking aportion of a metered amount of gauging water for use as the water.
 17. Awater-resistant gypsum panel having a core comprising interwovenmatrices of calcium sulfate dihydrate crystals and a silicone resin,said interwoven matrices having dispersed throughout them a catalystcomprising magnesium oxide and components from a Class C fly ash. 18.The panel of claim 17 further comprising at least one of the groupconsisting of starches, foaming agents, set accelerators, set retarders,biocides, dispersants, fibers or strength enhancers dispersed throughoutsaid interwoven matrices.
 19. The panel of claim 17 wherein said core issandwiched between two pieces of facing material.